1. Field of the Invention
The present invention is directed to a method for separating particulate materials of different density.
2. Description of the Prior Art
In the manufacture of metal powder, contamination from crucibles and other molten metal handling equipment and from the melt stock itself is a frequent problem. Generally, contamination is in the form of oxide particulate, such as SiO.sub.2, Al.sub.2 O.sub.3 and others. These contaminants are deleterious in that they adversely affect the mechanical and other properties of articles made from such metal powder. The problem is especially severe when the metal powder article is to be subjected to conditions of high stress and temperature where premature failure cannot be tolerated, such as a metal powder disc used to hold turbine blades in proper alignment in a gas turbine or "jet" engine. In making such articles, only the highest purity metal powder should be used.
A process commonly employed to separate one particulate material from another of different density is the so-called "sink-float" process (hereafter referred to as floatation process) wherein the particulate materials are placed in a liquid separation medium having a density intermediate to the densities of the materials so that the lighter material floats and the heavier material sinks. However, in utilizing this process to separate contaminants from metal powder, difficulties are encountered. First, the separation medium must not only have the desired intermediate density but also must be nonreactive with the metal powder. If the metal powder reacts with the medium, the powder may be contaminated by reaction product. Of course, this defeats the purpose of the floatation process. Second, high density liquids, such as thallium malonate and thallium formate, are employed in such processes and leave a liquid residue on the separated metal powder. This residue must be removed by washing, heating and other means to provide a final powder product of optimum quality which is ready for use in powder metallurgy fabrication processes, such as compaction, forging and the like.